This invention relates to quartz crystal sensors for monitoring the rate of deposition of thin films on a substrate. The invention is more specifically directed to improved electronic circuitry to sustain the oscillation of a piezoelectric quartz crystal and to measure its resonance frequency or frequencies.
Piezoelectric quartz crystals are commonly used to monitor the film thickness and to control the rate of film growth for vacuum deposited thin films The basic technique is described, e.g. in U.S. Pat. Nos. 4,817,430, 4,207,836, and 4,311,725. A multiple-crystal film thickness sensor is described in U.S. patent application Ser. No. 430,428, filed Nov. 2, 1989, which has a common assignee herewith. These monitors or sensors can be used both to monitor the vacuum deposition process and to control accurately the amount of material deposited and the rate of deposit onto a surface. The resonance frequency of the crystal drops with increasing thickness of the deposited film. As the film is deposited on the monitoring crystal, the sharpness of the resonance also degrades, and it becomes more and more difficult to distinguish the principal crystal resonance from other resonances and to measure its frequency precisely. That is, the natural resonances of any crystal depend upon total mass and geometry.
In these deposition monitors, there is typically one or more piezoelectric crystals which can be quartz, barium titanate, or other material. The crystals are connected into a resonant circuit so that the natural resonant frequency of one crystal can be monitored. The natural resonance frequency drops as material is coated onto the crystal. As the material builds up, the sharpness of resonance diminishes, and eventually a point is reached in which the crystal can no longer monitor the process accurately or effectively. The crystal must then be replaced. As the deposit on the crystal becomes thicker and thicker, the accumulated mass induces a change in the resonant frequency of the crystal and oscillator system. None of the previous or current monitoring circuits compensate or account for the density of the deposited material, nor do any of these monitoring circuits use the knowledge of the acoustic shear wave velocity to correct the behavior of the sensor circuitry to match the crystal. Commercially available process controllers based on the piezoelectric quartz crystal can make thickness measurements with a resolution smaller than one angstrom. These process controllers can use this information to make appropriate deposition rate corrections by changing the power to the deposition source. The power level can be changed as often as five to ten times per second. These crystals are extremely sensitive to added mass. A monolayer of copper added to a 6 MHz monitor crystal reduces its resonance frequency by roughly 20 Hz. To a first order approximation, the sensitivity is proportional to the deposited material density, and a material of higher density than copper will result in a sensitivity on the order of ten Hz per angstrom. The instrument performance is therefore determined by how well the frequency can be measured in a very short period of time. Any instability in the frequency of the oscillator/crystal system results in an inaccuracy of the measured film thickness. Because the thickness information is used in a feedback control system, measurement errors can cause the rate of deposition to become unstable. If the frequency instability is large it can also produce significant error in the final thickness of the film. Commercially available deposition process controllers obtain the resonance of the quartz crystal by utilizing a so-called "period" measurement technique. A stable, high-frequency reference oscillator is used as a time base to measure the time interval or period which is, of course, determined by the frequency of the monitor crystal. Any instability or lack of precision in the reference oscillator will be indistinguishable from a change in the resonant frequency in the monitor crystal. This will produce an error in the film's apparent thickness.
Measurement noise or random error can be reduced by using a longer time interval for greater smoothing due to averaging. Greater accuracy can also be obtained by designing a more stable reference oscillator to operate at a higher frequency.
Present deposition process controllers employ reference oscillators that range in frequency from 10 MHz to 225 MHz. The best of these have good enough stability to measure the monitor crystal frequency to a resolution of 0.2 Hz in an interval or period of 200 milliseconds. This corresponds to an error in the deposited film thickness of well under one angstrom for materials of average density.
While a one-angstrom error in final thickness in the deposited film is insignificant, measurement errors of fractions of an angstrom can be quite important in rate control, especially at low deposition rates. Feedback control stability for many types of evaporators requires that thickness measurements be taken many times per second. It is customary to measure the film thickness and update the control loop at least four times a second. Even more frequent control is often desirable.
At low deposition rates, the change in films thickness, and the corresponding change in crystal frequency between successive measurements is very small. For materials of low density, the change in the quartz crystal frequency from measurement to measurement is no greater than the measurement error from noise and reference oscillator precision. Thus, lack of precision can be a very serious problem in stable rate control at low deposition rates, because the controller cannot distinguish between frequency changes due to added material and frequency changes due to noise and/or uncertainty. When actual frequency changes are of the same order as the errors, the commands from the control system become erratic, and control over deposition rate is seriously degraded.
In the conventional measurement systems thus far described, there are two sources of uncertainty: measurement precision limitations and instabilities of the monitor crystal. The first limitations result from short sample times and from the stability limitations in the reference oscillator as previously described. The second limitations involve instabilities in the monitor crystal and in the active oscillator system, and these further degrade the measurement accuracy
Piezoelectric crystals are attractive as frequency stabilizing elements, in addition to their utility as deposition monitor transducers, because they possess a wide range of phase shift over a narrow range of frequencies. This property gives the crystal oscillator a great frequency stability, as the phase requirement for stable oscillation can be met over a quite narrow range of frequencies. This frequency stability can be achieved even in a very noisy environment because the phase component of the noise is cancelled out with a small change in frequency.
However, when the quartz crystal is used as a deposition monitor, this desirable phase/frequency relationship deteriorates as mass is added to the crystal. When the added mass from the deposition becomes significant, more frequency change is required to produce the same amount of phase shift. Any noise in the oscillator and crystal circuit will induce a significant frequency change that is not mass-related. This is a major cause of instability and deterioration of crystal performance near the end of the crystal's life.
Near the end of the crystal's useful life it may be unable to produce sufficient phase shift in the fundamental shear mode to satisfy the requirements for an oscillator. In this case another oscillation mode of the crystal may be better able to provide the required phase shift and gain. The aged crystal will cause the system to oscillate at that mode and a new frequency will result. Present measurement systems cannot avoid this change between oscillation modes and an associated instrument will infer erroneous thickness information. The change between these oscillating modes is called "mode hopping," and presents a serious problem in terms of rate control and final thickness cutoff.
If there is no other oscillation mode that will satisfy the phase and gain requirements for an oscillator, the system will fail to function altogether. Then the ability to measure thickness is lost. However in some cases the measurement system can sense that this has occurred and avoid generating false values. At that stage, a fresh crystal must be swapped for the existing one, but the process must be totally stopped so the vacuum chamber can be opened.